This invention relates generally to the field of gate valves and deals more particularly with an improved controlled float seat for an expanding gate valve.
In an expanding type gate valve, the gate mechanism is typically in the form of a gate member and an adjacent segment which cooperate with one another to expand tightly against the valve seats in both the open and closed positions of the valve. The gate mechanism collapses to facilitate movement between the open and closed positions without binding against the seats. In the past, it has been common for the valve seats to be pressed in place in a relatively fixed position in the valve body. As an alternative to the fixed seat, seats which are permitted to "float" to a limited extent have been devised, as exemplified by U.S. Pat. No. 4,116,419 to Diehl et al.
Such limited or controlled float seats have the advantage of being able to effectively seal against the gate and segment even if there is a lack of parallelism or if there are other irregularities such as are usually present in the surfaces of the gate assembly and valve seats. At the same time, the floating movement of the seats is controlled such that they do not drag excessively against the gate assembly when it is collapsed for movement between the open and closed positions. In the absence of a limitation as to the extent of movement permitted the valve seats, fluid pressure can press the upstream seat against the collapsed gate assembly with enough force to severely hamper its movement between the open and closed positions, thereby increasing the operating torque of the valve.